Increasing the number of levels of interconnects in integrated circuits provides additional routing capabilities, more compact layouts, better circuit performance and greater use of circuit design within a given integrated circuit surface area. A particularly useful level of connection is commonly called local interconnection, where neighboring diffused areas are connected to one another, and to neighboring polysilicon and metal lines.
One local interconnection method is disclosed in U.S. Pat. No. 4,675,073, issued on Jun. 23, 1987. As disclosed therein, the desired local interconnect is formed by patterning the residual titanium compound, for example titanium nitride (TiN), from the direct reaction forming titanium silicide (TiSi.sub.2) cladding of the diffusions and polysilicon gates. The titanium nitride is sufficiently conductive so that is useful to make local interconnections between neighboring regions. The disclosed process uses carbon tetrafluoride (CF.sub.4) as the reactant in a plasma etch to remove the undesired titanium nitride faster than titanium silicide.
An improved local interconnection method is disclosed in U.S. Pat. No. 4,793,896, issued on Dec. 27, 1988, and U.S. Pat. No. 4,863,559, issued on Sep. 5, 1989. Here a plasma etch using carbon tetrachloride (CCl.sub.4) as the etchant is used to etch the titanium nitride anisotropically. The preferred method is to perform the etch with a substrate temperature on the order of 50.degree. C.
Several problems exist with the current art. This chemistry achieves selectivity by the mechanism of polymerization. However, the control of the polymerization is very poor, leading to an unstable process. The control is poor, since the physical processes in the plasma are manipulated to determine reaction pathways and product distributions. Polymer deposition is controlled by substrate temperature, helium flow, carbon tetrachloride flow, power, pressure, reactor configuration, reactor material and other factors. Due to the large number of factors, the process can easily shift without significant reactor care. Moreover, the process latitude is very small due to the large number of strongly interactive parameters and the difficulty discharging carbon tetrachloride gas.
In addition, due to their chemical similarity, it is difficult to etch TiN faster than TiSi.sub.2. Further, the TiN:TiSi.sub.2 etch rate ratio decreases with etch time. Also, the dry etch process is difficult to install and maintain in a reactor, since the parameter domain required to achieve selectivity to silicide is small and sensitive to hardware configuration and change.
Another problem exists because the titanium residing on silicon oxide (field and sidewall) reacts with the silicon oxide at high temperature to form an interfacial material thought to be comprised of TiSi.sub.x O.sub.y. Since this material is a "hybrid" between silicon oxide and titanium silicide, it is difficult to etch, especially when located along the sidewall oxide of a polycide gate. This film is typically removed by means of a wet etch. This approach, however, has a number of shortcomings. The silicide selectivity is not adequate to remove the conductive filaments without unacceptably increasing the silicide sheet resistivity. Also, control of the wet etch is marginal, since it is depleted after about two hours. In addition, the wet etch attacks resist with the liability of exposing patterned TiN, and even the TiN patterned film is undercut at the silicon oxide/TiN interface.
Beyond the above limitations, another significant problem exists. Upon plasma ignition, polymer deposits over the whole wafer, inhibiting the etch. At elevated temperatures, as preferred in the prior art, in conjunction with ion bombardment, the polymer removal rate increases, so that etching initiates. However, the plasma power density is very high, typically 1 watt/cm.sup.3, so the wafer temperature increases with increasing etch time. As such, the net polymer deposition:removal ratio gradually decreases, reducing the etch selectivity during the later phases of the etch. Therefore, it is very difficult to control the process when the etch rates change with time. Moreover, the etch resist will often "burn" or reticulate at the elevated temperatures, making it more susceptible to lift-off during the dry etch and wet etch, used to clear the sidewall material, possibly eliminating the feature or decreasing the critical dimension control of a very small linewidth.